A transmission type liquid crystal mask marker is adapted for printing various static images on a surface of a workpiece by applying a high-density energy beam, such as a laser beam, to a transmission type liquid crystal mask (referred to as "a liquid crystal mask" hereinafter unless otherwise stated) on which the various static images are displayed, so as to apply to the workpiece surface the beam which has passed through a static image. In general, only static images for a mark are displayed on the liquid crystal mask of such a transmission type liquid crystal mask marker.
One of the inventors of the present invention has previously made a proposal for maintaining the printing performance of a transmission type liquid crystal mask marker (refer to Japanese Patent Laid-Open (a)6-39577. This proposal (referred to as "first proposal" hereinafter) has been made in consideration of the facts that the light transmittance of a transmission type liquid crystal mask increases as the temperature increases and deterioration proceeds, thereby decreasing printing accuracy, and that the light transmittance of a transmission type liquid crystal mask can be controlled by regulating a driving voltage. In this proposal, a light emitting element and a light receiving element are added to a conventional transmission type liquid crystal mask marker (if required, a liquid crystal mask temperature sensor is also added), and the following program is also provided on a controller. Namely, light from the light emitting element is applied to a static test image on the liquid crystal mask, and the transmitted light is received by the light receiving element so that an actual light transmittance Q is computed from the intensity of the emitted light R1 and the intensity of the received light R2. The applied voltage for a static image is regulated so that the actual light transmittance Q is equal to an optimum light transmittance Qo at the operation temperature. It is thus possible to maintain the printing performance of various conventional transmission type liquid crystal mask markers.
However, the first proposal also has a new problem with respect to matching with a transmission type liquid crystal mask marker which has recently appeared and which has the performance of high-accuracy and high-speed continuous printing.
The assignee of the present invention has also proposed a transmission type liquid crystal mask marker comprising a YAG laser mask marker (refer to Japanese Patent Laid-Open (A) 5-42379). This proposal (referred to as "second proposal" hereinafter) comprises a first XY deflecting device and a second XY deflecting device, which are provided in front of and behind a liquid crystal mask, respectively. A controller previously stores each of the divided static images of the whole image which is divided, and successively displays the divided static images on the liquid crystal mask. A laser beam scans the divided static images by the first XY deflecting device, and the laser beam passed through each of the divided static images is deflected by the second XY deflecting device and applied to the workpiece surface to complete the entire printing. As a result, high-accuracy and high-speed continuous printing is achieved.
However, a transmission type liquid crystal mask marker such as the second proposal, which enables high-accuracy and high-speed continuous printing, has difficulties in timing (time matching) with the first proposal. Namely, it is necessary to investigate a new problem with respect to timing.
On the other hand, a transmission type liquid crystal mask marker, e.g., a YAG laser mask marker, is sometime provided with one of various sensors serving as a light receiving element. A silicon photodiode is frequently used as a light receiving unit of the liquid receiving element in consideration of the cost, quality, temperature characteristics and compatibility with a light emitting element. As shown in FIG. 6, the sensitivity A of the silicon photodiode within the wavelength region (1.06 .mu.m) of a YAG laser is lower than the sensitivity within a shorter wavelength region (for example, refer to A. Yariv: Basis of Photoelectronics, P. 400, Maruzen (1988)). For example, when a semiconductor laser (wavelength of about 800 nm) or a He--Ne laser (wavelength of 633 man), which has high energy density, is used as the light emitting element, the sensitivity of the YAG laser is as low as about 1/10 of the sensitivity of such a semiconductor laser. Therefore, when a short-wavelength laser is used as the light emitting element, the effects on the light receiving unit can be relatively decreased, thereby preventing the problem with respect to a S/N ratio (signal/noise ratio), even if the YAG laser beam is scattered.
However, in a transmission type liquid crystal mask marker comprising a YAG laser using a Q switch or a pulse oscillation YAG laser, a laser pulse has high peak power. Although this peak power depends upon the operation conditions of the YAG laser or the structure of the YAG laser mask maker, the peak power is generally not less than 1 kW. The intensity of the scattered YAG laser beam, which is applied to the light receiving unit, is increased depending upon the arrangement of the light receiving unit of the light receiving element. As a result, noise occurs in the sensor output (the output of the light receiving element) due to the YAG laser beam, and the S/N ratio of the detection system of the light receiving element is thus decreased, thereby making unstable the operation of the YAG laser mask marker and deteriorating printing accuracy. In an extreme case, there are the problems that a sensing operation is made impossible, and that an error occurs in the operation of the YAG laser mask marker.